NEWTON, Massachusetts—The temperature stalls stubbornly at 32 degrees under a gray sky that drops the occasional snowflake. But a handful of runners gamely brave the chill to gather on the track behind a local high school.

One brings her oldest, who is home sick from school; the six-year-old is barely visible from inside a bundle of coats and blankets. Another arrives late from a parent-teacher conference that went long.

But they’re determined to be here—new mothers trading in their diaper bags for pink and flowery water bottles and seeking the escape of running alongside other new moms, no matter the weather or the competing obligations.

“Every moment other than this is focused on doing something else for other people,” says one, Eva Barnes, who has an 11-month-old daughter and finds time to train even though she’s back at work. “This is me doing this for me.”

The number of new mothers joining running clubs like the one that meets behind the high school, which is called Fit Mamas, is skyrocketing. Moms Run This Town, for instance, one of the preeminent national networks of clubs for new moms, has grown from 20 active chapters to more than 700 with an estimated 10,000 members just since 2011, says Pam Burrus, whose title with the group is head mama.

One reason is that the birth rate began last year to rebound after five years of declines, according to the U.S. Centers for Disease Control. Another is that there are more women runners overall; 57 percent of finishers of road races last year were women, up from 25 percent in 1990, the industry association Running USA reports. And elite runners such as Kara Goucher, who write on social media about balancing motherhood with running, serve as prominent role models.

But new mothers who run say their principal motivations are to get back in shape, restore their energy, combat post-partum depression, and hang out with fellow adults.

“We all need to not necessarily be just identified as being mothers,” says Burrus, speaking while her one-year-old naps. “We need our own identities, and running gives us that identity. It gives us mental clarity and a chance to blow off steam.”

Even when the stress intensifies, says Barnes, “and you feel like you can’t keep your head above water managing a full-time job and a baby and a relationship, you realize that some time to yourself improves your ability to manage all of those things.”

Some women runners can’t wait to start again. Erin Flynn, a 20-year competitive runner and former Saucony sales rep who quit when she had a baby last year and now coaches Fit Mamas, tried to run again two weeks after childbirth.

It was a mistake, she says now. But, “I was definitely that person who wanted to get right back to where I was. I needed to get out. I felt really isolated. It was an outlet.”

By her third child, who was born four months ago, Patience Shriber was also anxious to resume running. She got started again, with her doctor’s permission, after five weeks.

“I missed my body. I just wanted my body back,” she says.

But Shriber says that, for new mothers, running is about more than staying in shape. It’s essential to their mental health.

“I’m in a much better place when I work out,” she says. “It’s my only time to myself.”

“It’s nice to be around people who are in the same position,” she says. “It’s my alone time and I just get out and I don’t think about all the things I need to get done.”

None of these observations are frivolous, says Jamie Allison, founder and CEO of Moms in Motion, which has grown from 14 women in Santa Barbara to 45 chapters nationwide whose members pay national dues and, often, coaching fees.

“You’re sleep deprived. You’re still working. You’re going through something of an identity transition,” Allison says. “It’s just hard, no matter how much you like being a mom.”

“It does become a need that they look forward to every week—to meet up with other women who have similar lives, to learn from each other, to vent, to lift each other up when things aren’t going so great,” Allison says. “Running is the glue.”

The top reason new mothers run, according to a 2011 study at the University of North Carolina, Wilmington, is social motivation.

“All of a sudden you have the chance to meet up with these like-minded women,” says Dimity McDowell Davis, a mother of two and coauthor with Sarah Bowen Shea of Run Like a Mother, Train Like a Mother, and the forthcoming Tales from Another Mother Runner, out in March. “You’re enveloped by a sense of just belonging and not feeling like the wacko mom who had two hours of sleep last night and is wondering, ‘What have I done to my life?’”

It’s good for their kids, too. Research published in the journal Pediatrics found that more active mothers have more active children.

That makes running worth it, even on that cold and snowy morning on the high school track, Greta Feinberg says during a break from the tough Fit Mamas workout.

“I’ll think, ‘I gave birth to a baby,’” says Feinberg, a mother of two. “I can get through this.”